The present invention relates generally to seat rings and, more specifically, to an improved seat ring and valve assembly having improved alignment features, reduced torque requirements, and reduced leakage.
Conventional globe style control valves and other types of valves commonly include screwed-in or clamped-in seat rings. Seat rings are typically inserted into a cylindrical cavity formed within a valve body of a valve and include an annular thrust-bearing rim for receiving a valve plug to close and open the valve. The seat ring is usually a separate part so it can be replaced. Several economical materials can be used to construct seat rings, including steel, stainless steel, as well as hardened materials such as stellited, ceramic, and Tungsten carbide. The clamped-in seat ring typically requires a compressed gasket to seal the seat ring and prevent fluid leakage. The clamped construction typically requires additional parts and complexity due to the load that is needed in the valve assembly to compress the gasket. Screwed-in seat rings, on the other hand, generally do not require a gasket, but rely on a metal-to-metal sealing surface formed by a line of contact between the seat ring and the valve body. However, conventional screwed-in seat rings suffer certain alignment disadvantages and seal limitations as described below.
FIG. 1 is a cross-sectional view of prior art seat ring valve assembly 10, which includes seat ring 100 screwed into an interior cylindrical sidewall surface 122 of valve body 120. Seat ring 100 includes annular thrust-bearing rim 102, which receives valve plug 160 to open and close the valve. When valve plug 160 is in the closed position the valve plug is compressed against the seat ring 100 and prevents fluid from flowing through the passage 152 created when the valve plug is lifted from the seat ring. The rim 102 further includes exterior cylindrical sidewall 112, which forms a 90 degree angle (from horizontal as shown) and faces interior cylindrical surface 138 of valve body 120 (described in more detail below). Just below the rim 112, a tapered exterior surface, as shown, mates with a tapered surface 136 of the valve body 120. Both tapered surfaces are typically manufactured to be approximately 45 degrees. Together, the tapered surfaces and the sidewalls form the primary sealing surface or line of contact 106 between the parts to form the seal. Below this sealing surface, a threaded portion 104 of the seat ring 100 mates with a threaded portion 110 of valve body 120, which helps secure and guide seat ring 100 into cavity 130. The threaded portion also provides a secondary seal.
The effectiveness of primary line contact seal 106, however, is highly dependent on the alignment (meaning straightness and centeredness) of seat ring 100 within cavity 130. If seat ring 100 is misaligned, a tight uniform circular seal will not occur because certain sections of the seal will have gaps, particularly where there are minute surface flaws in the metal, resulting in unacceptable leakage between seat ring 100 and valve body 120. Unfortunately, conventional screwed-in seat ring assemblies of this type are particularly susceptible to misalignment resulting in leakage for at least three reasons: i) conventional screwed-in seat rings rely on their mated threads to provide alignment and a secondary seal, but given the tolerance between normal industry standard threads, the threads do not provide ample concentricity to achieve consistent shut-off; ii) the sharp edge formed between sidewall surface 112 and angular surface 132 of the rim forms a very narrow line of contact sealing surface during misalignment with valve body surface 136, which is susceptible to minute surface imperfection or irregularities; and iii) the relatively narrow angle (45 degrees) of valve body surface 136 does not sufficiently guide seat ring 100 down into the center of cavity 130, causing seat ring 100 to be susceptible to misalignment. In other words, a misaligned seat ring is particularly susceptible to leakage because it is either not properly centered, not straight, or both, causing at least a portion of the seal to be susceptible to leakage where there is insufficient contact between the opposing angles. To compensate for these limitations and to reduce leakage to an acceptable level, the assembler must apply an unacceptably high level of torque to seat ring 100 to excessively compress seat ring 100 within cavity 130. The required excessive compression creates a complicated assembly process, causes torque stress on the assembly parts, and contributes to a high failure rate of primary seal 106.
What is needed is an improved seat ring valve assembly having self-aligning characteristics for creating a stronger seal with reduced leakage without requiring an unacceptably high level of torque during assembly. Misalignment or lack of concentricity will also effect the ability for the valve plug 160 to seal against the seat ring 100.
An improved seat ring, valve body, and seat ring valve assembly have self-aligning characteristics that reduce leakage without the need for an unacceptably high level of assembly torque. The improved seat ring is screwed into a cavity formed within a valve body and includes a rim for receiving a valve plug used to open and close the valve. In order to prevent unacceptable leakage, a point of contact is provided by the seat ring to seat into an angled seating surface of the valve body which results in a superior primary contact seal. By providing a point of contact that seats on an angled seating surface, less torque is necessary than is otherwise needed for prior art seat ring valve assemblies. These and other features and advantages can be seen from the following drawings, specification, and claims.